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rlcZONE Products for the week of December 15, 2008
Osram Opto Semiconductors Says…
Next-Generation MultiLEDs From OSRAM
Black package for more contrast, white package for greater brightness
OSRAM Opto Semiconductors is launching two new MultiLEDs for different LED video displays. The new MultiLED in the jet black package has been developed specially for high-resolution displays. As the blackest LED currently on the market it provides excellent contrast and impressive depth of color. The new MultiLED in the white package offers exceptional brightness. It is perfect for high-resolution perimeter advertising systems or in displays with high brightness requirements. Both versions offer such complete color fidelity across the entire viewing angle that there are no discernible color differences even in the horizontal direction.
With its MultiLED in the black package OSRAM is targeting the professional entertainment industry, including theater shows and trade fairs. The LED offers particularly impressive contrast in video displays with a typical luminance of 2000 cd/m² and a pixel spacing of 5 to 7 mm. The color black appears on the display as a really deep black, which gives pictures great depth and creates 3D effects. "Achieving a consistent black level has been the greatest hurdle so far in presenting the color black properly. Pictures will now have much more intensive depth to them," said Dr. Volker Härle, LED Marketing Director at OSRAM Opto Semiconductors.
The white point for the MultiLED in the black package is at 750 mcd; the white point for the MultiLED in the white package is at 2000 mcd. The two versions are based on high-power chips in the latest Thinfilm and ThinGaN technology. They are supplied in very narrow tolerances in terms of brightness and color location to ensure homogeneous display results. The display colors remain constant from all viewing angles. The extremely bright multichip LEDs also benefit from their 6-lead design. This ensures good heat removal and enables currents of 15 mA per color to be achieved even at an ambient temperature of 85°C.
EN-Genius Says…
Osram Opto Semiconductors offers a unique product with its MultiLED packages, and one that lends itself to a variety of applications, not just video. In operation, the LED chips comprising the assemblies can all be controlled separately, which enables the display of all colors from one package, including white when all three chips are properly driven at the same time. Driving each LED chip individually lets you mix colors. This is also what achieves RGB-color equivalency with brightness control.
Osram’s 6-lead packages are also thermally stable, along with low operating temperature that spells reliability, which is especially important in the high-end products that will likely use these devices. Each assembly is silicone encapsulated, with a product lifetime spec of up to 50,000 hours. That spec depends on operating conditions, of course, but it's still a big plus.
Optical efficiency is 43 lm/W for red, 36 lm/W for true green, and 11 lm/W for blue. You can also operate the individual LED chips in serial or parallel. Because you get access to each anode and cathode, it's also possible to throttle the dissipation of an illumination system.
Sexy Applications
The Osram Opto Semiconductors MultiLED opens a lot of intriguing application possibilities. In addition to prosaic applications such as ambient lighting, can you imagine a full-color video wall? Indeed, these devices are attractive for video displays because they use black printed surfaces and diffused resin construction, which is what Osram claims dramatically increases contrast. Moreover, the use of thin-film technology and thin GaN (gallium nitride) provides extreme brightness, as well as the aforementioned optical efficiency.
Osram envisions the use of the MultiLEDs in a host of consumer-oriented products. In addition to video, these include kiosks and gambling machines, ultra-bright LCD backlights, general decorative and entertainment lighting, automotive interior lighting, and backlighting of automotive dashboards.
Enhancing LCDs
For backlighting, these RGB surface-mount devices are especially significant for LCDs, where they promise several advantages over CCFLs (cold cathode fluorescent lamps). For starters, their color range extends beyond the NTSC standard, http://ntsc-tv.com/ntsc-main-01.htm being about 50% greater than CCFLs. With CCFLs, the red component of their radiated light is fairly low.
Moreover, you can compensate for blurring effects by pulsing these LEDs. OSRAM believes that when very fast switching LCDs become available in the near future, color filters in these LCDs will be entirely eliminated by the likes of MultiLEDs. In its scheme, color filtering will be achieved directly by means of sequential color excitation, using arrays of MultiLEDs.
A Real World Test
In an actual backlighting test application at Osram Opto Semiconductors, a commercially-available 19-inch TFT (thin film transistor) monitor was used, with its conventional light source modified with MultiLED backlighting. The remainder of the original product, including its plastic housing, its light guides, and its optical films, were left unchanged.
The monitor’s two existing CCFL tubes, however, located along the sides of the display's light guide, were removed. Each CCFL was replaced with a strip of IMS-PCB (insulated metal substrate printed circuit board) containing 77 Osram MulitLEDs.
For the entire backlight, 154 LEDs were used, on a pitch of 5 mm. Osram contends that, in the future, fewer LEDs would be needed due to increases in brightness of the LED chips as processes improve. At some point in the future, the ratio of red, green, and blue might also be adjusted according to the type of LEDs used and their brightness and wavelength groups.
In any case, Osram's test monitor resulted in a backlight that was only 10-mm thick, and passively cooled by ventilation slots in the housing. The IMS-PCBs were also mounted on thin heatsinks. For each PCB strip, a group of eleven red, seven green, and seven blue LEDs were driven in series, so that a supply voltage of just 24 V could be applied. Current-limiting resistors were also used. Fine adjustment of RGB ratios was accomplished by means of pulse-width modulation.
A Foundation For The Future
It’s likely these kinds of applications signal just the beginning of a sea change in lighting. Because of their low intensity, LEDs as light sources for backlights have been limited to smaller displays. But, the rapid advancements in semiconductors like Osram’s MultiLEDs undoubtedly will lead to rethinking how mechanical packages can be designed. It’s fair to conclude that higher informational densities of next-gen displays will demand higher intensity, and more functional, backlight subsystems.
Moreover, CCFLs, due to the materials used and their construction, are sensitive to mechanical stress and vibration. Their use in automotive, aero, or military applications is either limited or impossible. In comparison, MultiLEDs can produce light within a narrow spectrum, and with much higher mechanical stability.
In addition, MultiLED lifetimes can be as great or greater than CCFLs. What's more, Osram MultiLEDs can be driven with simpler control circuitry and at low voltage levels. Another advantage is their nanosecond switching time specs. But perhaps the most significant advantage of these multi-color LEDs (as opposed to white LEDs and CCFLs) is the larger color range available, and with individually selectable so-called white points.
As mentioned in its press release, Osram MultiLEDs with black packaging offer the contrast needed in video displays. With MultiLEDs, the color black appears on the display as a truly deep black, which OSRAM claims will give pictures depth and create 3D effects. Heretofore, that has been a barrier to LED deployment in solid-state video displays.
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